In JP-2004-225598-A and its associated US-2004/0164187-A1 and DE-2004003361-A1, disclosed is a fuel injection system, which is used with an internal combustion engine (hereinafter simply referred to as “engine”) including a plurality of branch ports per one combustion chamber, for injecting fuel into intake air flowing through each branch port. According to a technique disclosed in JP-2004-225598-A and its associated US-2004/0164187-A1 and DE-2004003361-A1, two jets of fuel mist formed by the fuel injection valve are distributed to two branch ports. As such, the fuel injection valve forms two jets of fuel mist, thereby allowing the fuel to be less adhered to the wall portion which provides a partition between the two branch ports.
However, according to the technique disclosed in JP-2004-225598-A and its associated US-2004/0164187-A1 and DE-2004003361-A1, the fuel injection valve is installed in the branch port away from the combustion chamber. Accordingly, depending on the shape of the branch port, the fuel injected from the fuel injection valve may possibly adhere to the wall surface which forms the branch port. Furthermore, in a fuel injection system with one combustion chamber communicating with a plurality of intake ports, the fuel injection valve may be installed upstream of the branch point of the intake ports. In this case, the fuel injected from the fuel injection valve adheres to the partition wall installed in between the intake ports. As such, the fuel having adhered to the wall surface which forms the intake port flows into the combustion chamber without being sufficiently atomized. The insufficiently atomized fuel is not burned in the combustion chamber, and thus emitted from the engine as uncombusted hydrocarbon (HC). This leads to an increase in uncombusted HC in the exhaust gas and degradation in fuel consumption.
In this context, to prevent fuel from adhering to the wall surface which forms the branch port, the fuel injection valve may be conceivably installed in each intake port. On the other hand, the intake air distributed to an intake port further flows into the combustion chamber by way of a branch port. Accordingly, the flow quantity of the intake air flowing through the branch port is reduced. As such, to install a fuel injection valve in each branch port, the amount of fuel injected from the fuel injection valve needs to be reduced corresponding to the low flow quantity of intake air in order to facilitate the atomization of fuel. However, a reduction in the amount of fuel injected from each fuel injection valve installed in the intake port does not suffice the flow quantity of fuel required to increase the output of the engine.
Furthermore, as disclosed in JP-2003-262174-A, JP-2003-262175-A, and JP-2004-232463-A, the port injection type fuel injection system is designed such that the fuel injection valve is installed on the center axis of an intake valve for opening and closing the end opposing the combustion chamber.
In an engine with a plurality of intake valves, the end portion of the intake port opposing the combustion chamber is branched into two or more branch ports corresponding to each intake valve. Thus, when a fuel injection valve is installed in each of the branch ports branched from the intake port, the fuel injected from the fuel injection valve is biased due to the intake air flowing through the branch port.
For example, when one intake port is branched into two branch ports, the branch ports are branched from the intake port generally in the shape of a letter “Y” and curved toward the inner peripheral wall of the housing which forms the cylinder. Accordingly, the intake air flowing into the combustion chamber from the intake port by way of the branch port is formed closer to the inner peripheral wall of the cylinder. As a result, when fuel is injected from the fuel injection valve installed on the center axis of the intake valve, the fuel mist is carried on the intake air flowing through the branch port toward the inner peripheral wall. As such, the fuel injected from the fuel injection valve may readily adhere to the inner peripheral wall of the housing which forms the cylinder.
The fuel having adhered to the inner peripheral wall of the housing takes the form of droplets to be emitted out of the engine as uncombusted hydrocarbon without contributing to combustion in the combustion chamber. This may possibly cause an increase in hydrocarbon emitted from the engine and degradation in fuel consumption.
Also disclosed in JP-2000-234579-A and its associated U.S. Pat. No. 6,308,684 B1 is a fuel injection system, which is used with an engine including a plurality of intake ports per one combustion chamber, for injecting fuel into intake air flowing through each intake port. In the case of this technique, two jets of fuel mist formed by the fuel injection valve are distributed to the two intake ports. As such, the fuel injection valve forms two jets of fuel mist, thereby allowing the fuel to be less adhered to the wall portion which provides a partition between the two intake ports.
When two or more intake ports are in communication with one combustion chamber, the intake ports may have different inner diameters, the intake valve installed in each intake port may have different amounts of lift, and the flow quantity of intake air flowing through each intake port may be different. In these cases, according to the technique disclosed in JP-2000-234579-A and its associated U.S. Pat. No. 6,308,684 B1, the injection holes of the fuel injection valve are asymmetrically arranged to set the distribution ratio of fuel to be injected into each intake port.
However, some recent engines may stop opening and closing any one of a plurality of intake valves or change the amount of lift, for example, depending on the load of the engine. At this time, the flow quantity of the intake air flowing through each intake port varies depending on the amount of lift of the intake valve. According to the technique disclosed in JP-2000-234579-A and its associated U.S. Pat. No. 6,308,684 B1, fuel can be distributed to each intake port but the distribution ratio of fuel to be injected into each intake port cannot be changed. Accordingly, when a variation in the flow quantity of the intake air flowing through each intake port occurs due to a change in the load of the engine, part of the fuel injected from the fuel injection valve may stay in the intake port. The fuel staying in the intake port does not contribute to combustion in the combustion chamber. This leads to degradation in fuel consumption of the engine. Furthermore, the fuel staying in the intake port flows as in the liquid state into the combustion chamber when the intake valve is opened. Accordingly, the fuel is incompletely burned, thereby causing an increase in uncombusted hydrocarbon (HC) to be emitted from the engine.